Can't transfer data from external hard drive

Hi,

I recently started having problems with my external hard drive (Seagate Expansion 1TB), after 2-3 years of flawless performance. It started off having really slow copy speeds (kept dropping to around 3kb/sec), but my computer could read it and can show me what's on the drive (folders, files etc) however, I could not open them, copy them to the fixed hard drive or copy anything to the external hard drive. Now I plug it in and my computer has a bit of trouble reading what's on the disk, and when it can read it, there are some folders I can't even open (see error message below).

I get different error messages from both opening and copying files. For opening files/folders, I get:
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E:\PC Backups is not accessible.

The request could not be performed of an I/O error.

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Whilst for a copy/paste job, I get:
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An unexpected error is keeping you from copying this file

Error 0x8007085D: The request could not be performed because of an I/O device error.

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I've gone into a couple of PC shops where they told me that they had never encountered this problem before, but might be caused by a bit of dust getting on the disk itself and would suggest buying a new hard drive. I have also had a look at what error 0x8007085D is and it seems that it's a simple disk read error (most commonly due to smudged discs, according to the forums I've seen). I don't mind having to buy a new external hard drive, but I don't want to lose 600GB-worth of backups and important files I need.

Any ideas as to what could I do?

Many thanks

Adam