For home use, what is the best way to deal with viruses that could be on external storage, which seems like a great way for hackers to persist on your system even after a reformat. Maybe the best practice is to just not store file types that are often hacked like PDF, flash, etc? Or is Dropbox and Google Drive good enough at detecting malicious files?
Is this your own home machine? If it's a work network I would just block Google drive, Dropbox etc. People tend to take things home and work on them but means you have no control over the company data.
With that said I looked around a bit and it really depends on the provider some scan some don't and leave it to you to have a scanner installed. So I would just do some research on a provider before you use their services, at least that way you'll know where you stand.
I found the following in the security section about Google Drive.
Virus scanning: Google Drive scans a file for viruses before the file is downloaded or shared. If a virus is detected, users can't share the file with others, send the infected file via email, or convert it to a Google Doc, Sheet, or Slide, and they'll receive a warning if they attempt these operations. The owner can download the virus-infected file, but only after acknowledging the risk of doing so.
Only files smaller than 25 MB can be scanned for viruses. For larger files, a warning is displayed saying that the file can't be scanned.
I didn't find much on Dropbox except the Dropbox help page.
Dropbox will sync any files added to it, so if someone syncs a virus or malware file, it will be synced to any computers linked to the account. Other users' accounts and computers may also be affected if the virus or malware is in a shared folder. Malware files are generally not activated unless you click on them to open them. For this reason, just as with files from any other source, we recommend that you follow good security practices, such as running anti-virus software and exercising due caution when running unknown files from other computers.
I also found another question related to this Does Dropbox offer any protection against infected files?