Using optical media for offline backup is becoming more and more time consuming and expensive due to the increasing amount of data. Are there any alternatives these days besides using hard disk and solid state drives?
LTO tape comes to mind, but new equipment is priced sky-high, not for the consumer market.
There are bargains around on older, refurb lower capacity equipment though, which would still fit the needs for many.
I use multiple duplicate hard drive copies for backup. They aren’t run all that much so the failure rate for these drives are very low. SSDs might be an alternative depending on your capacity needs and might be more reliable being solid state.
Like what equipment?
What hard disk drives do you use?
No direct experience but lots of discussion on the datahoarder subreddit favors this approach.
Random sample:
Not what I would call bargains at those prices.
Compared to list 
remember these are enterprise market devices.
I use major brands like Toshiba, WD etc. I opt for drives that spin at 5,300 rpm over 7,200 rpm because I figure the slower spin rate increases reliability. Also, the speed difference between them for backup isn’t all that noticeable. They make drives that see a lot of use like recording security video non stop which might be more reliable. If you are going to use a stand alone enclosure then get a good quality one with a good power supply. These days I have switched to 2.5" drives for backup for convenience and that they now come in larger sizes. I know at some point I will move to SSDs.
I think most 2.5" hard disk drives these days are SMR and not CMR. What are the ones that you use?
I don’t know. The ones I use were bought 2-4 years ago. I plan to move to SSDs soon. I maintain three hard drive backups of active files at any given time. Once a project is completed it is transferred from the cloud server to a hard drive and cloud storage. I use Google for my working server but I also archive to a separate cloud storage. I have been backing up data for nearly thirty years without any loss. I started out using tape which I hated with a passion.
I have switched to a system of SSD’s for my main drives, (Crucial MX500’s),
Enterprise grade 7200 rpm HDD’s for backup with Macrium Reflect,
and BD-R’s for archiving backups that I still want to save but no longer refer to frequently enough to keep on the HDD’s. OWC periodically has sales on Toshiba Enterprise grade HDD’s.
I have stayed with Verbatim BD-R’s over the years and split my backup file volumes at 22GB so they will fit on the optical media.
HTH
If its only for casual backups then you could do much worse than the WD Elements USB3 HDD’s
Available up to a capacity of 5TB. Reasonably priced and reasonably fast.
If you want something a bit more robust, then consider either buying or building a NAS.
Much more expensive to buy but can be cheap if you can repurpose and old PC.