You should be able to use virtually any fat16 or fat32 flash drive. I've tried it with my digital camera's 128MB SD card, and both formats worked with it. Also, my 16MB ARMax Evo's memory stick (formatted in fat16, it's too small for fat32) works with it too. Dunno if that helps any.
As always, make sure to backup any data you have on your 512MB stick before trying it, cuz if it doesn't work, it could possibly corrupt your flash drive, making you need to reformat it.
Model: NTSC SCPH-39001/R (think it's a v7)
Mod: PS1 Exploit Softmod
Harddrive: Internal 40GB Seagate ST340810A
External drive: Apple iPod (in Disk Mode)