External Hard Drive Problem

[#1]

Not sure if this is a software problem or a hardware problem.
I have a large capacity hard drive which was being used as a normal hard drive in my desktop pc, but I had problems with it booting unreliably (sometimes it would, sometimes it wouldn't), so I replaced it with a new disk, got my pc up and running again reliably. The old hard drive I put into a case I had for building your own external hard drives, to see if it would work reliably as a backup store, which it has. Therefore, I am not sure why it had gone wrong in the pc. However, it is okay so I will continue with it in its present role.
My problem is I would like to reformat this old disk and change the Partion sizes on it, but Partition Magic 8 seems to have problems seeing it - Windows Explorer doesn't have that problem. Any suggestions?
Thanks in advance

[#2]

Booting hits some specific sectors very hard and once they fail you're pretty much stuffed.
As a data storage drive failing/failed sectors can be reallocated/bypassed.
I'd have a look at the SMART stats to be sure, but it could well run happily for years more (or not...).
If Windows will handle it why not just repartition using Disk Management?
Depending on the system it came from it may have a supplier-specific modified partition table that PM doesn't like, especially if it's an older version.
I found this comment online re PM8:

Quote

Cons: The software is very dated and a new version is long overdue.

[#3]

I think hard drives can become erratic if the computer power supply (PSU) is being overstretched.  I have certainly had boot issues that I have cured by replacing my PSU with a more powerful one.  I know that does not offer an obvious explanation for why switching to USB worked, but I still think it is a possibility.

[#4]

Its interesting to know your experience RR, and if I encounter the boot problem again, I shall try that.
As for the repartition and reformat, HP's answer worked. I had to use Windows Help to find out how to get at Disk Management, and the frst time I used DM to reformat the first partion, it both failed to do the job and it lost the patition letters on both partitions on the disk. Turning the disk off and restarting it cured that. I then did a quick format on the second partition and that worked fine, so I repeated the reformat on the first and that worked too.
I still needed to use PM8 on the external disk to combine the two partitions, and PM8 worked this time, so job done.
Thanks to both for your help