Does the 2009 iMac boot Windows using legacy BIOS or UEFI? Probably legacy BIOS. You can check the disk to see if it has legacy BIOS boot code in the MBR.
Here's an example disk.
Code:
joevt@Joes-Giga-Mac ~ % diskutil list /dev/disk1
/dev/disk1 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *240.1 GB disk1
1: EFI EFIB2 209.7 MB disk1s1
2: Microsoft Basic Data Windows 8.1 on Electra 109.2 GB disk1s2
3: Microsoft Basic Data Windows 10 on Electra 130.7 GB disk1s3
joevt@Joes-Giga-Mac ~ % sudo gpt -r show -l /dev/disk1
Password:
gpt show: /dev/disk1: Suspicious MBR at sector 0
start size index contents
0 1 MBR
1 1 Pri GPT header
2 32 Pri GPT table
34 6
40 409600 1 GPT part - "EFIB2"
409640 2008
411648 213237753 2 GPT part - "Windows 8.1 on Electra"
213649401 255212689 3 GPT part - "Windows 10 on Electra"
468862090 5
468862095 32 Sec GPT table
468862127 1 Sec GPT header
joevt@Joes-Giga-Mac ~ % sudo fdisk /dev/disk1
Disk: /dev/disk1 geometry: 29185/255/63 [468862128 sectors]
Signature: 0xAA55
Starting Ending
#: id cyl hd sec - cyl hd sec [ start - size]
------------------------------------------------------------------------
1: EE 0 0 2 - 25 159 6 [ 1 - 411647] <Unknown ID>
2: 07 25 159 7 - 1023 254 63 [ 411648 - 213237753] HPFS/QNX/AUX
*3: 07 1023 254 63 - 1023 254 63 [ 213649401 - 255212689] HPFS/QNX/AUX
4: 00 0 0 0 - 0 0 0 [ 0 - 0] unused
joevt@Joes-Giga-Mac ~ % sudo dd if=/dev/disk1 count=1 | xxd
1+0 records in
1+0 records out
512 bytes transferred in 0.000299 secs (1712375 bytes/sec)
00000000: 33c0 8ed0 bc00 7c8e c08e d8be 007c bf00 3.....|......|..
00000010: 06b9 0002 fcf3 a450 681c 06cb fbb9 0400 .......Ph.......
00000020: bdbe 0780 7e00 007c 0b0f 850e 0183 c510 ....~..|........
00000030: e2f1 cd18 8856 0055 c646 1105 c646 1000 .....V.U.F...F..
00000040: b441 bbaa 55cd 135d 720f 81fb 55aa 7509 .A..U..]r...U.u.
00000050: f7c1 0100 7403 fe46 1066 6080 7e10 0074 ....t..F.f`.~..t
00000060: 2666 6800 0000 0066 ff76 0868 0000 6800 &fh....f.v.h..h.
00000070: 7c68 0100 6810 00b4 428a 5600 8bf4 cd13 |h..h...B.V.....
00000080: 9f83 c410 9eeb 14b8 0102 bb00 7c8a 5600 ............|.V.
00000090: 8a76 018a 4e02 8a6e 03cd 1366 6173 1cfe .v..N..n...fas..
000000a0: 4e11 750c 807e 0080 0f84 8a00 b280 eb84 N.u..~..........
000000b0: 5532 e48a 5600 cd13 5deb 9e81 3efe 7d55 U2..V...]...>.}U
000000c0: aa75 6eff 7600 e88d 0075 17fa b0d1 e664 .un.v....u.....d
000000d0: e883 00b0 dfe6 60e8 7c00 b0ff e664 e875 ......`.|....d.u
000000e0: 00fb b800 bbcd 1a66 23c0 753b 6681 fb54 .......f#.u;f..T
000000f0: 4350 4175 3281 f902 0172 2c66 6807 bb00 CPAu2....r,fh...
00000100: 0066 6800 0200 0066 6808 0000 0066 5366 .fh....fh....fSf
00000110: 5366 5566 6800 0000 0066 6800 7c00 0066 SfUfh....fh.|..f
00000120: 6168 0000 07cd 1a5a 32f6 ea00 7c00 00cd ah.....Z2...|...
00000130: 18a0 b707 eb08 a0b6 07eb 03a0 b507 32e4 ..............2.
00000140: 0500 078b f0ac 3c00 7409 bb07 00b4 0ecd ......<.t.......
00000150: 10eb f2f4 ebfd 2bc9 e464 eb00 2402 e0f8 ......+..d..$...
00000160: 2402 c349 6e76 616c 6964 2070 6172 7469 $..Invalid parti
00000170: 7469 6f6e 2074 6162 6c65 0045 7272 6f72 tion table.Error
00000180: 206c 6f61 6469 6e67 206f 7065 7261 7469 loading operati
00000190: 6e67 2073 7973 7465 6d00 4d69 7373 696e ng system.Missin
000001a0: 6720 6f70 6572 6174 696e 6720 7379 7374 g operating syst
000001b0: 656d 0000 0063 7b9a 4aa1 1748 c37c 0000 em...c{.J..H.|..
000001c0: 0200 ee9f 0619 0100 0000 ff47 0600 009f ...........G....
000001d0: 0719 07fe ffff 0048 0600 f9bf b50c 80fe .......H........
000001e0: ffff 07fe ffff f907 bc0c 913c 360f 0000 ...........<6...
000001f0: 0000 0000 0000 0000 0000 0000 0000 55aa ..............U.
It has an EFI partition and two Windows partitions listed in the GPT.
The gpt command says "Suspicious MBR" which indicates that the disk probably has a hybrid MBR/GPT setup.
The fidisk command shows the MBR contains only the two Windows partitions. The protective partition (type EE) usually covers the entire disk in a GPT disk but here it only covers the blocks for the GPT partition table and the EFI partition. The protective partition hides the GPT partitions from Windows.
The dd code shows that that there is BIOS boot code in the MBR. Normally, the GPT has a MBR with mostly zero bytes except for the protective MBR partition.
To change the size of the partitions, I think you need to boot from a separate disk. You can connect an external USB or FireWire disk and boot from that. Of you can connect your iMac to another Mac using FireWire and boot the iMac in FireWire Target Disk Mode.
Then you can use a utility like iPartition.app to shrink your existing Mac partition and add a new partition. You might be able to shrink the Windows partition, but Windows might not like that. I'm not sure. You might need to boot the Windows CD and repair the BCD. Or maybe there's a utility to fix Windows for you. twocanoes makes Winclone which may help with a resized Windows partition or moving it to another disk.
Note that Legacy BIOS Boot only works with the iMac's internal drives so Windows would need to remain there.
I made a script to help with legacy boot issues.
https://gist.github.com/joevt/a99e3af71343d8242e0078ab4af39b6c
Run it before and after making changes to see what's happening to your disks and to verify that what you wanted to happen did happen.
