I'm in no way an expert on chipped Xbox's BIOS, so I can not say what has gone wrong now.
You needed to use a BIOS of the correct size for your chip that supports LBA48 ie. 137+GB HDDs. It also matters what Xbox version you're using too. A v1.6 needs specific BIOS or you'll get problems. HeXEn should have been the tool used to flash the BIOS correctly but looking at my copy I see it only includes 256K EvoxM8+ versions. Whether these can be flashed to 512K banks and work correctly I have no idea.
What my Xecuter 2.6 chipped Xbox uses I do not know for sure because I had it professionally chipped and did not think to ask for the information. I've been told 512K Evox M8Plusv16.fc.137 is almost definitely the BIOS it is using from my description and just from the fact it is a v1.6.
I later realised it has another, different BIOS on the other bank and AFAIK for it to work with a v1.6 and support 137+GB HDDs which it does it must(?) be 512K x2.5035.v16+.137. As far as I've tested it behaves identically. What the differences are between the BIOS versions available I've never seen any post or accompanying readme that explains their features, which you should use, why and how. The most you get is change log with the assumption you know what you're doing.
What I do know is that I've had no problems with the WD 320GB HDD I use in it now since I formatted it correctly using Chimp261812. Initially I'd made the mistake of using the older Chimp2618 which works fine with HDDs up to 249GB, the 16KB cluster size limit. When I eventually transferred more than 250GB to it the HDD corrupted immediately and I lost a huge amount of stuff on the F:\drive.
I had to start from scratch again using the right partitioning tools this time.
This thread I found seems relevant to your problem. I hope it is OK to post this link:-
https://www.reddit.com/r/originalxbo...a48_not_found/
JCRocky5's comments there about this sort of issue and his suggestions are likely very reliable.
Your original post reads to me as if there was actually nothing wrong except that the F:\drive had not been correctly formatted. As it is over 250GB it must be set up to use 32KB clusters not the default 16KB used for C:\, E:\, X;\, Y:\ and Z:\.
I would not have used the now ancient Slayers disc to re-format or anything else. I do not know what partitioning formatting tool it uses or what your version HeXEn uses for that job either.
The only recommended tools now are either XBPartitioner v1.3 or Chimp261812. Both guarantee correct cluster size for F (and G if used) depending on their size. Chimp has the much more noob friendly GUI thanks to Rocky5's work on it.
Also remember that an HDD sold as 320GB is actually 320,000,000,000Bytes. Its real size (using true base 1024) is therefore just over 298GiB. With the C:\, E:\, X;\, Y:\ and Z:\ drives using <10GiB your F:\ drive will therefore be a maximum of about 288GiB.