[CentOS] Disk Performance Issue

Mon Jun 24 15:42:24 UTC 2019
Chris Olson <chris_e_olson at yahoo.com>

We have a very old Dell desk top machine that has been running
CentOS 6 for the past five years.  It received a new, 1 TB disk
and additional memory before the OS installation.  It has been
the primary Linux machine in our smallest and most remote field
office.  It has been updated at least once a week and has all
current dates installed.

Boot-up this morning lasted about six times as long as usual.
Disk access, as indicated by the disk activity light, is almost
continuous and for extended periods of time when ever something
is done that requires the disk.  Everything observed happens
whether or not the machine is connected to our network. All of
our files appear to be accessible if one is patient.

One theory put forward is that some application is running that
uses up CPU and disk bandwidth.  Another theory is that thereare disk errors, mostly corrected by EDCS features. We do not
see any rogue applications and error logs show no disk issues.

This is a mysterious issue that we hope to circumvent by putting
a new disk and installing CentOS 7 from DVD.  Our hope is that
the current disk can be mounted externally on the new CentOS
system using a USB to SATA adapter and that data can be moved
off of the old disk. 

Advice regarding this issue and any possible diagnostic methods
will be greatly appreciated.

df -k output:
Filesystem        1K-blocks      Used Available Use% Mounted on
                   51475068  12110896  36742732  25% /
tmpfs               1928152       176   1927976   1% /dev/shm
/dev/sda1            487652    211073    250979  46% /boot
                  905124888 246856176 612284356  29% /home