I know that RAID 5 allows for 1 drive to fail, but I can't find information if the data on the failed drive is accessible while the array is degraded. Would you still be able to access all data with 1 drive missing or is the data on the failed drive inaccessible until the array is rebuilt?
The array will still be fully functional (except redundancy) with one drive down. However, if you lose a second drive before a new drive is rebuilt, the data will be lost.
RAID 5 requires that all drives but one be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost.
@Steven's answer is correct, and it answers the second question in the OP.
Yes - all your data is available with one failed drive.
But the other question is whether you can access data on the failed drive.
The answer is no. A failed drive is a filed drive. You can't access anything from it.
-->The Question implies that the RAID array will need to be rebuilt. That is not exactly the case. When the failed drive is replaced it needs to be added to the array and then the system will include it while still running.
In other words, all data is available the entire time except for the short power off to install the new drive, unless you have hot-plug capability.
With one failed drive your RAID5 system needs a replacement for the failed drive. It is working OK, but by being degraded like this it means that one more failed drive will take out the whole array. At that point it would need to be rebuilt.
Takeaway: Make a good backup of everything in the array immediately. Then replace the failed drive.
Yes, you should still be able to access your logical drive normally while one physical drive in a RAID5 is missing. However, you should plug in a replacement of the same size or greater as soon as possible. The same model is best. Performance will be degraded while it rebuilds and fills the new drive with data. I've only ever used hot-plug RAID (meaning I don't have to unmount the logical drive in order to rebuild it), but your RAID controller might not allow it. Check the specific manual for your RAID controller to be certain.