PC is a Dell Precision T3600. It has various I/O which include: 1 x serial, 2 x PS2. I'm not using PS2 (mouse & KB are wireless)

excerpt from dmesg is:
[ 3.338990] ..............
[ 17.599314] input: PS/2 Generic Mouse as /devices/platform/i8042/serio1/input/input4
[ 18.123017] ..............................................................................not responding...
[ 103.038229] sd 0:0:1:0: [sda] 976773168 512-byte logical blocks: (500 GB/466 GiB)

Looks like it's hanging for 85 secs. Question is how do I fix it.

Just for testing purposes, have you tried booting with a USB mouse (or even a PS2 mouse if you happen to have one)?

Searching on that I see lots of posts about track-pads being identifies as a PS/2 mouse, so it might be something like that. But it is probably not causing your boot pause of 85 seconds.

Try running to see what is holding up the boot process,assuming you are running a distro with systemd.

Thanks utech and frankbell for your replies.

More grist for the mill .....

$ systemd-analyze
Startup finished in 1min 45.637s (kernel) + 10.978s (userspace) = 1min 56.616s
graphical.target reached after 10.971s in userspace

$ systemd-analyze blame doesn't identify anything of any significant duration, viz:

6.839s NetworkManager-wait-online.service
2.253s systemd-udev-settle.service
1.117s udisks2.service
719ms dev-sda3.device
430ms systemd-journal-flush.service
405ms logrotate.service
etc...

I don't have a USB/PS2 mouse to hand - but worth a shot when I can find one.

I'll keep digging.

You might have a dead battery in the mouse.