Is 5MB/s a reasonable speed for Dell Software Raid Controller in Raid5

That is absolutely not acceptable performance for a RAID setup, software or hardware. An individual SATA drive would outperform that by many times. Could you provide more detail around what kind of drives you are using? Are you sure your array isn't already running with a failed disk, in a degraded state? I also know that using "green" SATA hard drives can cause problems in RAID arrays due to their built-in power management.

To answer your symptom of a problem question, check the System event log for disk and file system errors. Failures to read or write in a timely enough fashion will be logged.

You can pull a single drive at a time out of a RAID-5, but while the disk is removed your array is performing in a degraded state, and you will have to replace the drive and rebuild completely before removing another drive. If you remove a second drive before the first drive rebuilds, you will cause the array to fail. Be careful not to do this with a live disk if you already have a failed disk, as I referred to previously as a possibility.

I have an LSI 8308 ELP SAS/SATA hardware RAID controller in a lab box with 4 old 250 GB SATA-II drives installed on it in a RAID-5, and it reads/writes at around 300 MB/sec, and I have 2 SATA-II SSDs on my workstation on a software RAID (ICH10R) that reads/writes 500 MB/sec. In my case, these are both either very legacy or consumer-class devices that are outperforming your server by 60-100x.