External hard drive on mac os takes 5 minutes to be recognised, whereas on windows pc its instantly. recently its not worked at all - help?

each OS alters the drive when ejected properly (before you unplug it).

Windows tends to handle this faster than MacOS does.

then again, if your Mac is older, it may not have USB 3.0 ports, which affects speed.

3.0 ports are usually blue.

the mac partition has a few bad(slow) blocks, the deep down reading algorithm will try many many reads for each sector to attempt to get REPEATABLE good readings, thus creating a SLOW effect. there are many ways to fix this issue but the best is disk diagnostics which repairs the sectors. other options are(poor choice but does help a bit) is running Chkdsk/F on any windows partition. dont know if macs bother with that idea for a partition. ie its a file system correcting application that ALSO looks for bad(unreliable) sectors and can declare them BAD(but only within the file system table) which helps avoid using them. Ultimately only a proper diagnostics can repair them (rewrite their start and end markers and test them ) or assign them replacement (from a small set of spares) within the drive itself. you COULD also delete the offending partition and instead create two partitions in place of it where the 1st would be SMALL(say 10% of the space, and you would never format it or ever use it and we would assume it would contain the BAD sectors) and the 2nd would be say 90% of the space which you could re-format and hope that the bad stuff was back in the unused sectors. Note: windows CHKDSK can only repair a windows partition. the links below may help if you can get the mac up and running with the partition mounted.

You might want to try and reformat it. Make sure to get all the data off of it as formatting wipes that, then format it as FAT (32) for compatibility.