Is it safe to defrag external hard drives?

I have two external hard drives (WD brand), and as of a couple of days ago, one of them (don't worry, I know which one) has been really slow in loading up files. This has never happened before, and I think it may be due to them not being defragged. However, I want to make sure that defragging external hard drives won't cause any problems like file deletion, accidental formatting or anything like that. I've defragged the C: drive, but that didn't help. I have my external hard drives plugged in at ALL times. The only time I remove them is when I switch my PC off. Also, I'm running Windows XP Service Pack 2, and I've had this PC for over +10 years, but it has no WiFI so I can't upgrade it to Service Pack 3 (not that it matters).. Hopefully, this info will help in answering the question. I would really appreciate it.