If your backup solution synchronizes your local content with the cloud content, then yes, your cloud files can be just as corrupt as your local ones. It depends on your detection rate (how quickly you detect any file is corrupted by virus/malware or media error) and the rate of synchronization (how often do you synchronize your local data with your cloud data).
If your backup solution uses synchronization in combination with versioning, the time constraints on detecting corrupted files is less demanding, but still very important. At least you have an earlier version to fall back upon. Of course, this is moot when you don't detect file corruption quickly enough, because your good copy of the file is still overwritten, depending on the amount of file versions your backup solution can handle.
Now, you can cover your behind a bit by using incremental backups (also known as: deltas), that way you have a (hopefully good) base copy of your backed up files and the incremental backups will only have the altered files since the previous incremental backup, all the way back to your first backed up files. An incremental backup set is usually quite fast to make and doesn't require that much storage space. However, such a backup is useless without all the previous incremental backups and the original base copy.
Now you want to use portable hard disks, so your choice of media is ok enough regarding storage capacity and writing speeds. Still, it might not be such a bad idea to have a copy of the original base copy on write-once media, such as DVD-R or DVD+R or writable BluRay discs. And store these carefully in a controlled environment off-site. The rationale is that no virus or malware can alter your base copy on these discs, because of their write once functionality.
With all of the above in mind, you must define for yourself how important the data you are backing up is to you. For very important data (for example: digitized photo's or tax returns) it could be a good idea to do all of the above. Less important files you could only cover by incremental backups, etc, etc.
This is called a backup strategy. To execute such a backup strategy takes quite a lot of work and (self-)discipline. What is worse, you also need to make sure you can retrieve your backed up data. Something you really don't want to find out when you actually need to restore files. You can test by doing full restores in a virtual machine or bare-metal system you have laying around. This can be time-consuming task too.
You could save yourself quite a bit of time by selecting a random set of files in a backup set and restore these in a VM/bare-metal PC. If successful, you can believe that the remainder of files in the backup will be restorable too. you could also generate hash codes from the files you want to backup and verify these hash codes with the hash codes generated from the files you have restored. Hash code checking is a decent enough method to use in backup solutions that are highly automatized.
Any backup strategy must cover the questions:
- how important are the backed up files to me?
- how often do I want to create backups? (real-time, hourly, daily, weekly, etc)
- where do I store my backups? (safe, off-site, cloud, etc)
- how often do I verify the created backups
When you have defined that for yourself, you also have a much better idea about which software to use (including the level of automatization). I ended up using Bacula (open source enterprise solution), which can do all of the above, but it was really dreadful to setup. Afterwards it has proven to be very reliable. It allowed me to do full backups and incremental backups of both Linux and Windows PCs at varying synchronization rates completely automatically for on-site and off-site storage on hard disks and write-once DVDs.
Personally, I don't have much faith in cloud solutions for file storage, specifically here in Paraguay. To me, the cloud is only useful for quickly ramping up computing resources when needed. Anyway, that is completely different discussion. The only reason I mention this is that the cloud is not part of my backup strategy. But I am sure you can make Bacula work with the cloud as well, if you so desire.
Having said that, Bacula has a steep learning curve, is Linux-based and requires you to install a client on a Windows PC before you can use it to backup, so it is not everyone's cup of tea. Now I haven't looked at the back-up options included with Windows since Windows Vista came out, but I remember Fred Langa's reasonably positive comments about such options in his Windows Secrets newsletter contributions.