tooter, I had something of the same problem, I have a 120gb Maxtor hd, I had all games installed ok, I used faplink to try to install something, faplink failed the transfer, when I booted up hdloader the files were there so I deleted them and they deleated the whole hdd, so I thought hdloader wiped the whole hd, so I put it back into the PS2 and used winhiip, it told me that there was partions missing and if I wanted to recover them, I clicked yes and there is was, my whole game list.
So therefore, try putting the drive back into the PC and use winhiip see if there is anything it can do to recover the partions. as far as I know Seagate drives are 100% compatable, like the Maxtor drives, so there shouldn't be any reason why they wouldn't work
SCPH 50003 (V9) with Smartchip
120gb Maxtor 7200 HDD
V1.5 Xbox with Aladdin Advance M8 Bios